fix(xilinx): wire Xilinx platforms in docs

Wire Xilinx platform docs to fix build errors when
make DEBUG=1 V=1 doc is called.

Fixes: 30b8bc2aadb8 ("docs(versal-net): add Versal NET documentation")
Fixes: e52f311919f3 ("docs(versal): add versal documentation")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Change-Id: I52f4cf31bec2716a2f8b84a1f01335e387d39556
diff --git a/docs/index.rst b/docs/index.rst
index 4869af1..7e54db2 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -12,6 +12,7 @@
    design
    implementing-tests
    porting/index
+   plat/index
    change-log
    license